Ion-conducting membranes typified by ion-exchange membranes, polymer electrolyte membranes and the like are functional membranes having ion conductivity and electron insulating properties. Ion-exchange membranes include cation-exchange membranes having cation conductivity and anion-exchange membranes having anion conductivity. Cation-exchange membranes and anion-exchange membranes are widely used in the field of electrolytic industries based on the combination of properties of both the membranes, and are also beginning to be used in applications requiring high-quality membranes such as pharmaceutical manufacturing.
Recently, use of polymer electrolyte membranes having hydrogen ion conductivity and hydroxide ion conductivity is also expected to expand. Polymer electrolyte membranes are used in polymer electrolyte fuel cells that convert hydrogen or hydrocarbons into electric energy, hydrogen production devices that produce hydrogen from water, electrochemical hydrogen compressing devices and the like as a catalyst coated membrane including an electrolyte membrane and a catalyst applied or transferred to the electrolyte membrane, or a membrane electrode assembly including a catalyst coated membrane and electrodes attached to the catalyst coated membrane. For promotion of diffusion of fuel cells and utilization of hydrogen energy, not only quality improvement of polymer electrolyte membranes, but also a low-cost mass production method and a low-cost production device for polymer electrolyte membranes are desired.
Ion-exchange membranes and polymer electrolyte membranes that are ion-conducting membranes usually contain a polymer having an ionic group. Methods of introducing an ionic group into a polymer are roughly divided into a method of polymerizing a polymer using a monomer having an ionic group, a method of introducing an ionic group into a polymer by a polymeric reaction, and a method of forming a membrane of a polymer and then introducing an ionic group into the membrane-shaped polymer likewise by a polymeric reaction. Since the ionic group is in a state of a salt (ion pair) with a counter ion such as a metal ion or a halogen ion in the course of the synthesis reaction, in those methods, it is necessary to finally convert an ion-conducting membrane capable of exhibiting its functions by exchanging a metal ion with a hydrogen ion by acid treatment or exchanging a halogen ion with a hydroxide ion by alkali treatment. Hereinafter, a membrane that contains a polymer containing a salt of the ionic group with an impurity ion and a counter ion and that is in a state before being converted into an ion-conducting membrane by liquid treatment with an acid solution or an alkali solution is referred to as a “precursor membrane.”
In the production of an ion-conducting membrane by the above-mentioned method, any metal ions or halogen ions remaining as impurities in the ion-conducting membrane cause deterioration of ion conductivity and electron insulating properties as well as deterioration of durability. To reduce the concentration of impurity ions such as metal ions and halogen ions in the ion-conducting membrane, however, it is necessary to use a large amount of treatment solution in the liquid treatment, which prevents reductions in its production cost. As a technique to reduce the amount of use of the treatment solution, Japanese Patent Laid-open Publication No. 2013-56993 discloses, as a method of producing a polymer electrolyte membrane including, in acid treatment, immersing a precursor membrane in an acidic solution a plurality of times, a liquid treatment method for a hydrocarbon polymer electrolyte membrane including cascade-conveying a film to a plurality of immersion tanks filled with an acidic solution, and continuously supplying the acidic solution while overflowing the acidic solution in a cascade method in a direction opposite to the film conveying direction.
When immersing a precursor membrane in an acidic solution a plurality of times as described in JP '993, however, the immersion time is prolonged in proportion to the number of times of immersion, and the size of the liquid treatment tanks is also increased in proportion to the number of times of immersion.
It could therefore be helpful to provide a production method of producing an ion-conducting membrane with reduced impurities in a short time as well as a downsized device that produces an ion-conducting membrane capable of reducing the amount of use of the treatment solution.